Inorganic glass has been generally used in the field of an optical part such as a lens, and an optical material such as a liquid crystal display device, a color filter, an EL display device substrate and other display substrate, a backlight or a light guide plate. However, inorganic glass has such defects that it is fragile, lacks flexibility, has a large specific gravity and is poor in processability, and thus is unsatisfactory for satisfying demands for weight reduction, size reduction and densification in recent years. Accordingly, there is a strong demand for replacement thereof with a transparent resin.
Upon applying a transparent resin to the optical material field, significantly high performances are required in heat resistance, chemical resistance and low water absorption properties, in addition to transparency. For example, in the production of a display device substrate, which requires a high temperature processing step for laminating a metal or metal oxide thin film, deformation of the substrate under heat, dimensional change due to absorption of water, or the like causes problems. However, an acrylic resin and a polycarbonate resin, which have been used as an optical material, have defects of low heat resistance and large water absorption properties, and thus are not sufficient.
Under the circumstances, a cycloolefin addition polymer has been proposed as a resin that satisfies transparency, heat resistance, chemical resistance, low water absorption properties and optical properties, and a material for a liquid crystal display substrate using the polymer has been also proposed (Patent Document 1).
A cycloolefin addition polymer, particularly polynorbornene, has a high glass transition temperature of 250° C. or higher, and is therefore a material that is excellent in resistance to heat deformation upon processing at a high temperature. Furthermore, polynorbornene has such characteristics that it is excellent in dimensional stability on fluctuation in humidity in the use environment owing to significantly low hygroscopicity and that it is excellent in dimensional stability on fluctuation in heat owing to the low linear expansion coefficient of about 55 ppm.
However, polynorbornene has a problem that it cannot be formed into a film by a casting method due to low solubility thereof in a common solvent.
Accordingly, various improvements of polynorbornene have been studied, and (1) an addition copolymer with an α-olefin such as ethylene (Patent Document 2), (2) an addition copolymer with a norbornene substituted with a linear hydrocarbon group (Patent Document 3), (3) an addition copolymer with a norbornene compound having a specific cyclic saturated hydrocarbon group (Patent Document 4), (4) an addition copolymer with a norbornene compound having a polar group (Patent Documents 5 and 6), and the like have been proposed.
However, the norbornene/ethylene copolymer (1) has problems that it is difficult to produce a polymer having a glass transition temperature of 200° C. or higher and that the linear expansion coefficient is increased to about from 70 to 100 ppm. The addition copolymer with norbornene substituted with a linear hydrocarbon group (2) also has the similar problems of decrease in glass transition temperature and increase in linear expansion coefficient. The addition copolymer with a norbornene compound having a particular cyclic saturated hydrocarbon group (3) is impractical due to difficulty in the synthesis of the particular norbornene compound, and the solubility in a common solvent is not necessarily enhanced. The norbornene copolymer having a polar group (4) is exemplified by a copolymer with a norbornene compound having a long-chain ester group or a silyl group as a polar group, but it has a problem of an increased water absorption ratio, and also has a problem of a largely increased linear expansion coefficient.    Patent Document 1: JP-A-5-61026    Patent Document 2: JP-A-6-202091    Patent Document 3: JP-A-8-198919    Patent Document 4: JP-A-2004-51949    Patent Document 5: JP-T-11-505880    Patent Document 6: JP-A-2002-114826